Increased intraocular pressure is a frequent disorder of the eye which is often associated with optic nerve damage, in which case the disease is glaucoma. In the absence of optic nerve damage, the condition is referred to as ocular hypertension.
Normal intraocular pressure is usually defined as being in the range from 10 to 21 mmHg. The pressure results predominantly from balance between the production rate and the drainage rate of the aqueous humour in the eye. In addition, it is influenced by the corneal thickness and rigidity. The intraocular pressure typically fluctuates around about 15 to 16 mmHg with amplitudes of up to 6 mmHg. For example, it usually decreases in the night due to a decreased production of aqueous humour. It also responds to various physiological factors such as exercise, heart rate, respiration, fluid intake, as well as certain types of systemic or topical drugs.
The aqueous humour is produced by the ciliary bodies of the eye, from where it flows into the posterior chamber. The composition of the aqueous humour is very similar to that of blood plasma but differs from the latter by a lower protein content. Its main constituents are water (99%), electrolytes (inorganic ions to maintain the physiological pH), low amounts of albumin and β-globulins, ascorbate, glucose, lactate, and amino acids.
From the posterior chamber, the aqueous humour is distributed via the pupil of the iris into the anterior chamber of the eye. From here, it flows through the so-called trabecular meshwork, which is a spongy tissue area lined by trabeculocytes whose main function is to drain the humour into a set of tubes called Schlemm's canal, from where the humour enters the blood circulation. The humour flow from the trabecular meshwork into the Schlemm's canal occurs via two different routes: either directly via the aqueous vein to the episcleral vein, or indirectly via collector channels to the episcleral vein by intrascleral plexus. This trabecular outflow pathway accounts for the major fraction of drained aqueous humour. In addition, there exists a second major drainage pathway which is the uveoscleral outflow, which is relatively independent of the intraocular pressure and normally accounts for only 5 to 10% of the aqueous humour drainage in healthy humans.
Both in the trabecular meshwork and in the uveoscleral tissue, various prostanoid receptors have been found, which indicates that prostanoids are involved in the regulation of aqueous humour production and/or drainage and thereby influence the intraocular pressure. In the trabecular network, genes encoding the EP, FP, IP, DP and TP receptor families are expressed, whereas the EP and FP receptor families are dominant in the uveoscleral tissue (Toris et al., Sury Ophthalmol. 2008; 53, Suppl. 1, S107-S120).
Prostanoids are physiological fatty acid derivatives representing a subclass of eicosanoids. They comprise prostaglandins, prostamides, thromboxanes, and prostacyclins, all of which compounds are mediators involved in numerous physiological processes. Natural prostaglandins such as PGF2α, PGE2, PGD2, and PGI2 exhibit a particular affinity to their respective receptors (FP, EP, DP, IP), but also have some non-selective affinity for other prostaglandin receptors (ibid.). Prostaglandins also have direct effects on matrix metalloproteinases. These are neutral proteinases expressed in the trabecular meshwork which play a role in controlling humour outflow resistance by degrading the extracellular matrix.
Several prostaglandin analogues have been found effective as topically administered medicines in reducing the intraocular pressure, such as latanoprost, bimatoprost, tafluprost, travoprost and unoprostone. By some experts, bimatoprost is understood as a prostamide rather than prostaglandin derivative.
Latanoprost, travoprost, tafluprost and probably also bimatoprost are potent and selective PGF2α agonists. Their net effect is a reduction of intraocular pressure, which is predominantly caused by a substantial increase in aqueous humour drainage via the uveoscleral pathway. Probably they also increase the trabecular outflow to some degree.
Unoprostone is sometimes also classified as a PGF2α analogue even though its potency and selectivity are much lower than in the case of the previously mentioned compounds. It is most closely related to a pulmonary metabolite of PGF2α. It is also capable of reducing the intraocular pressure, but appears to act predominantly by stimulating the trabecular drainage pathway, whereas it has little effect on the uveoscleral outflow.
Various eye drop formulations comprising prostaglandin analogues have been developed and are commercially available. Latanoprost and travoprost are provided as buffered, isotonised, preserved aqueous solutions in multidose bottles having a strength of 50 μg/mL (0.005%) and 40 μg/mL (0.004%), respectively. Tafluprost is available in a similar preserved formulations as well as in a non-preserved formulation in single-dose containers. The tafluprost formulations have a strength of 15 μg/mL (0.0015%) and additionally contain the surfactant, polysorbate 80. Bimatoprost is also marketed as a buffered, isotonised, and preserved aqueous solution; its strength is 0.3 mg/mL (0.03%). The strength of the commercial unoprostone formulation is 1.5 mg/mL (0.15%). It contains buffer, a preservative, an isotonising agent, and polysorbate 80.
However, preserved aqueous formulations for ophthalmic use are disadvantageous in that they are capable of producing irritancies or hypersensitivity reaction, in particular in long-term use, such as in glaucoma therapy. The most common preservative in the formulations mentioned above is benzalkonium chloride, a quaternary ammonium compound which is associated with frequent irritant toxic reactions. Non-preserved single use containers avoid this disadvantage, but they are expensive. Not only do they require a container for each single dose, but also an overfill of the formulation, which means that a substantial fraction (if not most) of the actual medicine remains in the container and is discharged as waste. Considering the drug in an eye drop which is actually administered into the eye, only a fraction of that becomes effective due to the limited volume capacity of the lacrimal sac: a significant fraction of the administered fluid volume is expelled by the blinking of the eyelids, and another fraction is taken up systemically via the nasolacrimal duct, which potentially leads to adverse drug effects.
In spite of the preservative contained in the currently available formulation of latanoprost, there have been reports of bacterial keratitis caused by microbiological contamination of the product assumingly by the patients themselves, indicating that the microbiological safety of the product is only relative.
An alternative to aqueous eye drop formulations are oil-based ophthalmic compositions. They are often capable of better dissolving poorly water-soluble drug substances. Moreover, they do not normally require the incorporation of pH adjusting agents or isotonising agents.
One of the disadvantages of all oil-based formulations for ophthalmic administration is that inherently have a negative impact on vision. Whether used as oily solutions or oil-in-water emulsions, they exhibit a refractive index which differs substantially from that of physiological tear fluid, which leads to visual disturbances and blurring.
Moreover, oil-based formulations do not readily mix with tear fluid to form a homogenous liquid phase. Oily solutions are altogether immiscible with the aqueous tear fluid, and the exact fate of an emulsion mixed with tear fluid in a physiological setting is not completely predictable.
Oil-in-water emulsions of poorly water-soluble drugs like ciclosporin further exhibit the disadvantage that they have a limited drug load capacity. While the active ingredient may have some solubility in the oil phase, this phase is only dispersed in the coherent aqueous phase of the emulsion so that the maximum overall drug concentration in the formulation is very limited.
In contrast to single phase systems such as aqueous or oily solutions, oil-in-water emulsions are also more complex and difficult to manufacture, especially in sterile form. Frequently, emulsions are not readily sterilisable by thermal treatment without negative impact on the physical properties of the emulsion. On the other hand, aseptic processing is complex, costly, and is associated with higher risks of failure, i.e. microbial contamination of the product.
Furthermore, oil-in-water emulsions are like aqueous solutions prone to microbial contamination during use. If they were to be presented in multi-dose containers which are in principle more cost-efficient and convenient for patients than single-use vials, they would have to be preserved in order to ensure their microbiological quality. At the same time, preservatives which can be used in ophthalmic formulations are potentially irritating, as mentioned above, or even damaging to the eye.
WO 2005/123035 discloses hydrophobic compositions which may be useful as ophthalmic drug formulations. The compositions may be used to treat various ophthalmic diseases and conditions including glaucoma and may comprise a therapeutic agent selected from various different therapeutic categories such as antibiotics, antimicrobials, antifungal agents, antiviral agents, antiparasitic agents, antiallergic agents, anti-inflammatory agents, alkylating agents, prostaglandin analogues and beta-blockers, cholinergic agents, vasoconstrictors, pupil size management agents, glaucoma agents, macular degeneration agents, and agents to arrest the development of cataracts. The hydrophobicity of the composition is achieved by selecting a hydrophobic liquid vehicle, selected in particular from silicon polymers, fluorinated silicon polymers, perfluorocarbons, fluorinated alcohols, and perfluorinated polyethers, and mixtures thereof. However, the only specific composition disclosed in the document does not incorporate an active ingredient, but is merely a vehicle consisting of a mixture of two silicon polymers, namely dimethicone and cyclomethicone, which have been combined so as to yield a viscosity of about 8,000 centistokes.
US 2002/128527 discloses semifluorinated alkanes and their preparation, and proposes their use as vehicles in ophthalmic preparations. However, it does not disclose any specific compositions comprising a semifluorinated alkane and an incorporated active ingredient. Neither does it mention the treatment of glaucoma or the incorporation of a prostaglandin analogue. It is also silent about ophthalmic vehicles comprising mixtures of semifluorinated alkanes and cosolvents.
U.S. Pat. No. 7,026,359 describes the use of highly fluorinated oligomeric alkanes in ophthalmology. These highly fluorinated compounds, whose chemical structure is different from that of the semifluorinated alkanes referred to in US 2002/128527, represent a large group of hydrocarbons having 2 to 6 perfluorinated side chains. The document does not mention any specific composition comprising such a fluorinated compound and an active ingredient. In fact, it does not make any reference to any particular active agent at all, or even to any specific chemically or functionally defined class of active ingredients. Neither does it disclose any specific therapeutic indications within ophthalmology in which the use of the highly fluorinated compounds could be useful.
U.S. Pat. No. 5,874,481 is directed to thermodynamically stable molecular solutions of lipophilic active ingredients in mixtures of a lipophilic fluorochemical and a non-fluorinated cosolvent. While the patent in a very general manner refers to numerous classes of therapeutic compounds from which the active ingredient may be selected, including ophthalmic agents, it only provides very few specific composition comprising a selected active ingredient, namely solutions of diazepam, caffeine and prednisone, respectively, all of which are systemic rather than ophthalmic therapeutic agents.
It is an object of the present invention to provide a novel pharmaceutical composition which is useful in the treatment of increased intraocular pressure, e.g. in association with open-angle glaucoma or ocular hypertension, which these issues discussed above and overcomes at least one of the limitations or disadvantages associated with prior art formulations. In a specific aspect, it is an object of the invention to provide an ophthalmic composition which has the capacity to incorporate substantial amounts of poorly water-soluble drug substances useful in the management of open-angle glaucoma and/or ocular hypertension. In a further aspect, it is an object of the invention to provide a pharmaceutical kit comprising a composition for the treatment of increased intraocular pressure which does not exhibit one or more of the disadvantages of prior art. Further objects of the invention will become clear on the basis of the following description, examples, and patent claims.